1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a CMOS image sensor wherein a charge control device is installed between a photo sensing device and a reset transistor to discharge excessive charges, thereby preventing interference with an adjacent transistor resulting from the excessive charges.
2. Discussion of the Related Art
Generally, image sensors are semiconductor devices which convert optical images into electrical signals, and may be roughly classified into a charge coupled device (CCD) image sensor or a complementary metal oxide silicon (CMOS) image sensor.
In the CCD image sensor, a driving system is complex, a large amount of power is consumed and a multistep photo process is required to result in a complex manufacturing process. Further, it is difficult to integrate a controller, a signal processor, an analog/digital (A/D) converter, etc. in one chip, which leads to difficulty in miniaturization of a product. Recently, the CMOS image sensor has been proposed to overcome the disadvantages of the CCD image sensor.
The CMOS image sensor is based on a CMOS technology which employs a controller, a signal processor, etc. as peripheral circuits. In the CMOS image sensor, MOS transistors corresponding to unit pixels are formed on a semiconductor substrate and a switching system is adopted to sequentially detect outputs of the respective unit pixels through the MOS transistors. In the CMOS image sensor, a small amount of power is consumed and a small number of photo process steps are performed, resulting in simplification in the manufacturing process. Furthermore, it is possible to integrate the controller, signal processor, A/D converter, etc. in a CMOS image sensor chip, thereby facilitating miniaturization of a product.
However, in such a conventional CMOS image sensor, when the amount of incident light is too large, excessive charges may be stored in a photo sensing device that acts to convert an optical signal into an electrical signal. In this case, the charges stored in the photo sensing device may overflow to an adjacent electrical contact point, resulting in signal distortion.
This problem with the conventional CMOS image sensor will hereinafter be described in detail with reference to FIGS. 1 and 2.
FIG. 1 is a circuit diagram of a unit pixel of the conventional CMOS image sensor having four transistors. As shown in this drawing, the CMOS image sensor comprises a photo sensing device, for example, a photodiode 100 for generating photo charges; a transfer transistor 101 for receiving a Tx signal at its gate and transferring the photo charges generated by the photo sensing device 100 to a floating diffusion region FD 102 in response to the received Tx signal; a reset transistor 103 for receiving an Rs signal at its gate, and in response to the received Rs signal, setting the potential of the floating diffusion region FD 102 to a desired value and discharging charges stored in the floating diffusion region FD 102 to reset the floating diffusion region FD 102; a drive transistor 104 for acting as a source follower buffer amplifier in response to a signal input to its gate; and a select transistor 105 for performing an addressing operation in response to a signal inputted to its gate.
FIG. 2 shows an overflow phenomenon which occurs in the CMOS image sensor of FIG. 1.
When strong light is incident on the photo sensing device 100 formed in a P-well 111 on a P-type semiconductor substrate 110, the generation of a large number of charges resulting from the incident light occurs very rapidly. For this reason, the charges that should only be gathered in the photo sensing device 100 may be passed through a region beneath an STI 112 and then transferred to an adjacent transistor or photo sensing device, thereby causing signal distortion.